1. Field of the Invention
The present invention relates to flat display screens, and more particularly to so-called cathodoluminescence screens, the anode of which carries luminescent elements, separated from one another by insulating areas, and likely to be excited by electron bombarding. This electron bombarding requires the biasing of the luminescent elements and can come from microtips, from layers with a low extraction potential or from a thermo-ionic source.
2. Discussion of the Related Art
To simplify the present description, only color microtip screens will be considered hereafter, but it should be noted that the present invention generally relates to the various above-mentioned types of screens and the like.
FIG. 1 shows the structure of a flat color microtip display screen.
Such a microtip screen is essentially comprised of a cathode 1 having microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which constitutes the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, in particular, in U.S. Pat. No. 4,940,916 of the Commissariat a l'Energie Atomique.
Cathode 1 is organized in columns and is comprised, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. The microtips 2 are implemented on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field which is created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if the latter are adequately biased. In the case of a color screen, anode 5 is provided with alternate bands of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The bands are parallel to the cathode columns and are separated from one another by an insulator 8, generally silicon oxide (SiO.sub.2). The phosphor elements 7 are deposited on electrodes 9, comprised of corresponding bands of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue, bands are alternately biased with respect to cathode 1, so that electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed towards the phosphor elements 7 facing each of the colors.
The control for selecting the phosphor 7 (phosphor 7g in FIG. 1) which is to be bombarded by the electrons from the microtips of cathode 1 imposes to control, selectively, the biasing of phosphor elements 7 of anode 5, color per color.
Generally, the rows of grid 3 are sequentially biased at a potential of about 80 volts, while the bands of phosphor elements (for example 7g in FIG. 1) to be excited are biased under a voltage of about 400 volts via the ITO band on which the phosphor elements are deposited. The ITO bands, carrying the other bands of phosphor elements (for example, 7r and 7b in FIG. 1), are at a low or zero potential. The columns of cathode 1 are carried to respective potentials between a maximum emission potential and a no emission potential (for example, respectively 0 and 30 volts). The brightness of a color component of each of the pixels in a line is thus determined.
The choice of the values of the biasing potentials is linked with the features of phosphor elements 7 and of microtips 2. Conventionally, below a potential difference of 50 volts between the cathode and the grid, there is no electronic emission, and the maximum emission used corresponds to a potential difference of 80 volts.
A disadvantage of conventional screens is that they have a low lifetime, that is, after a relatively short operating time (of about one hundred hours), the brightness of the screen decreases significantly and destructive phenomena due to the forming of sparks between the screen cathode and anode may even sometimes be observed.
Further, after a certain operating time, the color appears to vary and no longer corresponds to the screen control settings. This phenomenon will be called herein "color shift". In practice, this means that at least one of the bands of phosphor material adjacent to the biased bands starts exhibiting a luminescence.
The origin of this phenomenon was not well understood up to now. It was thought to be due to the fact that electrons accumulate on the insulating areas 8 between the bands of phosphor material and ensure a conduction to neighboring bands. To avoid this phenomenon, several prior art techniques have been provided for, one of which consists in separating by short time intervals the biasings of the anode bands between two successive sub-color frames, and applying a negative voltage pulse on the band just biased, before positively biasing the next anode band to be excited.
However, this method has the disadvantage of being relatively complex to implement since it complicates the delivery of the anode supply voltages, which are voltages with high values (a few hundred volts) and it is prejudicial to the brightness of the screen.